Earths Dynamic Systems Hamblin

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Global Tectonics II-Origin of an Idea
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• Divergent Plate Boundaries
• Plates moving apart

• Convergent Plate Boundaries
• Plates moving toward one another

• Transform Plate Boundaries
• Plates move laterally past each other

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Cascadia Suduction Zone-local Plate Tectonics
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Continental Drift

• Predecessor to modern plate tectonics
• Shape and fit of the continents was the initial
  evidence
• Snider-Pelligrini (1858)
• Taylor (1908)
• Wegner (1915)

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Continental drift maps by Wegner (1915)
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Continental Drift

• Alfred Wegener (1880-1930)
• Proposed all of the continents were once part of
  a large supercontinent - Pangaea
• Based on
• Similarities in shorelines
• Distinctive rock and fossil groups found in
  Africa South America

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Evidence for Continental Drift

• Paleontological
• Similarity of fossils on opposite sides of the
  Atlantic and Indian Oceans

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Paleontological evidence
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Evidence for Continental Drift

• Rock type structures
• Distinct rock type and geologic structures on
  both sides of the Atlantic Ocean

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Rock type structure evidence
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Evidence for Continental Drift

• Glaciation
• Late Paleozoic glaciation
• Covered large portions of the southern continents
• Distinct glacial deposit

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Reconstruction from glacial deposits
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Evidence for Continental Drift

• Paleoclimate
• Evidence of extreme changes in climate as
  compared to the present

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Paleoclimate evidence
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Development of Plate Tectonic Theory

• Original evidence for continental drift was from
  continental rocks
• Technological advances in the 1950s and 1960s
  allowed investigation of the sea floor
• Geophysics paleomagnetism provided new data

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Geology of the Ocean Floor

• Topography of the ocean basins
• Basins are divided by a large ridge system
• Ridge system continuous around the entire globe
• Central rift valley within the ridge

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• Paleomagnetism
• Fe rich minerals magnetized by Earths magnetic
  field present at time of formation.

• Polar wandering
• Earths north magnetic pole has (apparently)
  moved through time

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Reconstruction from paleomagnetic data
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• Magnetic reversals
• Earths magnetic field polarity has reversed
  through time

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• Magnetic polarity stripes in ocean crust parallel
  ridges
• Symmetrical on either side of the ridge
• Polarity chrons give age of seafloor
• Increases away from ridge
• Rates of plate motion may be calculated

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Patterns of magnetic reversals
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Age of the sea floor
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Global Distribution of Earthquakes
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Major tectonic boundaries
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• Divergent Plate Boundaries
• Plates moving apart

• Convergent Plate Boundaries
• Plates moving toward one another

• Transform Plate Boundaries
• Plates move laterally past each other

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Divergent plate margins
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Convergent plate boundaries
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Rates of Plate Motion

• Two ways to look at plate motion
• Relative velocity the movement of one plate
  relative to another
• Absolute velocity compares plate movement to a
  fixed position
• Rates vary from 1 to 20 cm/yr

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Rates of plate motion around the world
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Tectonic Mechanisms

• Convection of heat from the core and mantle
  drives tectonics
• Convection cells bring new material to the
  surface
• Old crust is pushed away from ridges
• Subduction carries cool crust back into the mantle

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Models of plate tectonic motion